A conventional propelling system has been used in diverse forms according to the characteristics and usage of ships, and a screw propeller system is the most basically and generally used in the art.
A representative screw propeller system is a fixed pitch propeller system.
Referring to FIG. 1, a fixed pitch propeller system 1 is shown to include blades, integrally fixed to a hub, connected to a rotating shaft thereof, so that the hub and the blades are rotated all together when the rotating shaft, which is connected to an engine, is rotated.
Here, the flow around the blades upon rotation generates variation in pressure on the surfaces of the blades to create thrust in the advancing direction of a ship, the speed of which can be continuously varied with the control of revolutions per minute.
However, the conventional fixed pitch propeller system 1 generates a spiral streamline, which causes to contract the wake flow 4a in shape behind the propeller system. This reduces an efficiency of the propeller and therefore the thrust of a ship.
In particular, since the conventional fixed pitch propeller system 1 generates a swirl flow 3a, such as a swirling vortex, perpendicular to the direction of thrust 2, energy loss is caused, so that the thrust efficiency is somewhat degraded.
Meanwhile, there is proposed a counter-rotating propeller system to remedy the above disadvantage and improve efficiency.
Referring to FIG. 2, a conventional counter-rotating propeller system 5 uses a power transmission system 6 having a complicated dual-propeller shaft structure, in which two propellers, rotating in opposite directions, are mounted on one shaft.
For example, the propeller shaft includes a hollow shaft member and a solid shaft member inserted into the hollow shaft member. A large front propeller is mounted on a hub of the hollow shaft member and is rotated in a normal direction at the outside, whereas a small rear propeller, positioned inside, is mounted on a hub of the solid shaft member and is rotated in a reverse direction.
The hydrodynamic actions of the front propeller and the counter-rotating rear propeller minimize the swirling flow and recover the energy, thereby increasing the thrust 2 in an amount of recovery 3b. 
The conventional counter-rotating propeller system is widely used as a propeller system for a torpedo, which needs a straight running, since the propellers are rotated in counter directions relative to each other to achieve torque balance between the propellers. It has also been proved that, when also used in a general commercial ship, the conventional counter-rotating propeller system provides an improvement in thrust efficiency.
However, as set forth before, the conventional counter-rotating propeller system requires the dual-propeller shaft structure, the related power transmission system, and a special reverse-rotating device, which is not cost effective to be used in a general commercial ship. Thus, there are many restrictions in use because of the complexity and the excessive manufacturing cost of the counter-rotating system.
Further, the conventional counter-rotating propellers still have the above problems in that the wake flow 4b has a contracted shape, whereby the propeller efficiency is decreased and thus a thrust of a ship is lowered.